Struct moka::future::Cache [−][src]
pub struct Cache<K, V, S = RandomState> { /* fields omitted */ }
Expand description
A thread-safe, futures-aware concurrent in-memory cache.
Cache
supports full concurrency of retrievals and a high expected concurrency
for updates. It can be accessed inside and outside of asynchronous contexts.
Cache
utilizes a lock-free concurrent hash table SegmentedHashMap
from the
moka-cht crate for the central key-value storage. Cache
performs a best-effort bounding of the map using an entry replacement algorithm
to determine which entries to evict when the capacity is exceeded.
To use this cache, enable a crate feature called “future”.
Examples
Cache entries are manually added using an insert method, and are stored in the cache until either evicted or manually invalidated:
- Inside an async context (
async fn
orasync
block), useinsert
orinvalidate
method for updating the cache andawait
them. - Outside any async context, use
blocking_insert
orblocking_invalidate
methods. They will block for a short time under heavy updates.
Here’s an example of reading and updating a cache by using multiple asynchronous tasks with Tokio runtime:
// Cargo.toml
//
// [dependencies]
// moka = { version = "0.6", features = ["future"] }
// tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
// futures = "0.3"
use moka::future::Cache;
#[tokio::main]
async fn main() {
const NUM_TASKS: usize = 16;
const NUM_KEYS_PER_TASK: usize = 64;
fn value(n: usize) -> String {
format!("value {}", n)
}
// Create a cache that can store up to 10,000 entries.
let cache = Cache::new(10_000);
// Spawn async tasks and write to and read from the cache.
let tasks: Vec<_> = (0..NUM_TASKS)
.map(|i| {
// To share the same cache across the async tasks, clone it.
// This is a cheap operation.
let my_cache = cache.clone();
let start = i * NUM_KEYS_PER_TASK;
let end = (i + 1) * NUM_KEYS_PER_TASK;
tokio::spawn(async move {
// Insert 64 entries. (NUM_KEYS_PER_TASK = 64)
for key in start..end {
// insert() is an async method, so await it.
my_cache.insert(key, value(key)).await;
// get() returns Option<String>, a clone of the stored value.
assert_eq!(my_cache.get(&key), Some(value(key)));
}
// Invalidate every 4 element of the inserted entries.
for key in (start..end).step_by(4) {
// invalidate() is an async method, so await it.
my_cache.invalidate(&key).await;
}
})
})
.collect();
// Wait for all tasks to complete.
futures_util::future::join_all(tasks).await;
// Verify the result.
for key in 0..(NUM_TASKS * NUM_KEYS_PER_TASK) {
if key % 4 == 0 {
assert_eq!(cache.get(&key), None);
} else {
assert_eq!(cache.get(&key), Some(value(key)));
}
}
}
Thread Safety
All methods provided by the Cache
are considered thread-safe, and can be safely
accessed by multiple concurrent threads.
Cache<K, V, S>
requires trait boundsSend
,Sync
and'static
forK
(key),V
(value) andS
(hasher state).Cache<K, V, S>
will implementSend
andSync
.
Sharing a cache across asynchronous tasks
To share a cache across async tasks (or OS threads), do one of the followings:
- Create a clone of the cache by calling its
clone
method and pass it to other task. - Wrap the cache by a
sync::OnceCell
orsync::Lazy
from once_cell create, and set it to astatic
variable.
Cloning is a cheap operation for Cache
as it only creates thread-safe
reference-counted pointers to the internal data structures.
Avoiding to clone the value at get
The return type of get
method is Option<V>
instead of Option<&V>
. Every
time get
is called for an existing key, it creates a clone of the stored value
V
and returns it. This is because the Cache
allows concurrent updates from
threads so a value stored in the cache can be dropped or replaced at any time by
any other thread. get
cannot return a reference &V
as it is impossible to
guarantee the value outlives the reference.
If you want to store values that will be expensive to clone, wrap them by
std::sync::Arc
before storing in a cache. Arc
is a
thread-safe reference-counted pointer and its clone()
method is cheap.
Expiration Policies
Cache
supports the following expiration policies:
- Time to live: A cached entry will be expired after the specified duration
past from
insert
. - Time to idle: A cached entry will be expired after the specified duration
past from
get
orinsert
.
See the CacheBuilder
’s doc for how to configure a cache
with them.
Hashing Algorithm
By default, Cache
uses a hashing algorithm selected to provide resistance
against HashDoS attacks. It will be the same one used by
std::collections::HashMap
, which is currently SipHash 1-3.
While SipHash’s performance is very competitive for medium sized keys, other hashing algorithms will outperform it for small keys such as integers as well as large keys such as long strings. However those algorithms will typically not protect against attacks such as HashDoS.
The hashing algorithm can be replaced on a per-Cache
basis using the
build_with_hasher
method of the
CacheBuilder
. Many alternative algorithms are available on crates.io, such
as the aHash crate.
Implementations
Constructs a new Cache<K, V>
that will store up to the max_capacity
entries.
To adjust various configuration knobs such as initial_capacity
or
time_to_live
, use the CacheBuilder
.
Returns a clone of the value corresponding to the key.
If you want to store values that will be expensive to clone, wrap them by
std::sync::Arc
before storing in a cache. Arc
is a
thread-safe reference-counted pointer and its clone()
method is cheap.
The key may be any borrowed form of the cache’s key type, but Hash
and Eq
on the borrowed form must match those for the key type.
Ensures the value of the key exists by inserting the output of the init future if not exist, and returns a clone of the value.
This method prevents to resolve the init future multiple times on the same key even if the method is concurrently called by many async tasks; only one of the calls resolves its future, and other calls wait for that future to complete.
Example
// Cargo.toml
//
// [dependencies]
// moka = { version = "0.6", features = ["future"] }
// futures = "0.3"
// tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
use moka::future::Cache;
use std::sync::Arc;
#[tokio::main]
async fn main() {
const TEN_MIB: usize = 10 * 1024 * 1024; // 10MiB
let cache = Cache::new(100);
// Spawn four async tasks.
let tasks: Vec<_> = (0..4_u8)
.map(|task_id| {
let my_cache = cache.clone();
tokio::spawn(async move {
println!("Task {} started.", task_id);
// Insert and get the value for key1. Although all four async tasks
// will call `get_or_insert_with` at the same time, the `init` async
// block must be resolved only once.
let value = my_cache
.get_or_insert_with("key1", async move {
println!("Task {} inserting a value.", task_id);
Arc::new(vec![0u8; TEN_MIB])
})
.await;
// Ensure the value exists now.
assert_eq!(value.len(), TEN_MIB);
assert!(my_cache.get(&"key1").is_some());
println!("Task {} got the value. (len: {})", task_id, value.len());
})
})
.collect();
// Run all tasks concurrently and wait for them to complete.
futures_util::future::join_all(tasks).await;
}
A Sample Result
- The
init
future (async black) was resolved exactly once by task 3. - Other tasks were blocked until task 3 inserted the value.
Task 0 started.
Task 3 started.
Task 1 started.
Task 2 started.
Task 3 inserting a value.
Task 3 got the value. (len: 10485760)
Task 0 got the value. (len: 10485760)
Task 1 got the value. (len: 10485760)
Task 2 got the value. (len: 10485760)
Panics
This method panics when the init
future has been panicked. When it happens,
only the caller whose init
future panicked will get the panic (e.g. only
task 3 in the above sample). If there are other calls in progress (e.g. task
0, 1 and 2 above), this method will restart and resolve one of the remaining
init
futures.
Try to ensure the value of the key exists by inserting an Ok
output of the
init future if not exist, and returns a clone of the value or the Err
produced by the future.
This method prevents to resolve the init future multiple times on the same key even if the method is concurrently called by many async tasks; only one of the calls resolves its future (as long as these futures return the same error type), and other calls wait for that future to complete.
Example
// Cargo.toml
//
// [dependencies]
// moka = { version = "0.6", features = ["future"] }
// futures = "0.3"
// reqwest = "0.11"
// tokio = { version = "1", features = ["rt-multi-thread", "macros" ] }
use moka::future::Cache;
// This async function tries to get HTML from the given URI.
async fn get_html(task_id: u8, uri: &str) -> Result<String, reqwest::Error> {
println!("get_html() called by task {}.", task_id);
Ok(reqwest::get(uri).await?.text().await?)
}
#[tokio::main]
async fn main() {
let cache = Cache::new(100);
// Spawn four async tasks.
let tasks: Vec<_> = (0..4_u8)
.map(|task_id| {
let my_cache = cache.clone();
tokio::spawn(async move {
println!("Task {} started.", task_id);
// Try to insert and get the value for key1. Although
// all four async tasks will call `get_or_try_insert_with`
// at the same time, get_html() must be called only once.
let value = my_cache
.get_or_try_insert_with(
"key1",
get_html(task_id, "https://www.rust-lang.org"),
).await;
// Ensure the value exists now.
assert!(value.is_ok());
assert!(my_cache.get(&"key1").is_some());
println!(
"Task {} got the value. (len: {})",
task_id,
value.unwrap().len()
);
})
})
.collect();
// Run all tasks concurrently and wait for them to complete.
futures_util::future::join_all(tasks).await;
}
A Sample Result
get_html()
was called exactly once by task 2.- Other tasks were blocked until task 2 inserted the value.
Task 1 started.
Task 0 started.
Task 2 started.
Task 3 started.
get_html() called by task 2.
Task 2 got the value. (len: 19419)
Task 1 got the value. (len: 19419)
Task 0 got the value. (len: 19419)
Task 3 got the value. (len: 19419)
Panics
This method panics when the init
future has been panicked. When it happens,
only the caller whose init
future panicked will get the panic (e.g. only
task 2 in the above sample). If there are other calls in progress (e.g. task
0, 1 and 3 above), this method will restart and resolve one of the remaining
init
futures.
Inserts a key-value pair into the cache.
If the cache has this key present, the value is updated.
Blocking insert to call outside of asynchronous contexts.
This method is intended for use cases where you are inserting from synchronous code.
Discards any cached value for the key.
The key may be any borrowed form of the cache’s key type, but Hash
and Eq
on the borrowed form must match those for the key type.
Blocking invalidate to call outside of asynchronous contexts.
This method is intended for use cases where you are invalidating from synchronous code.
Discards all cached values.
This method returns immediately and a background thread will evict all the
cached values inserted before the time when this method was called. It is
guaranteed that the get
method must not return these invalidated values
even if they have not been evicted.
Like the invalidate
method, this method does not clear the historic
popularity estimator of keys so that it retains the client activities of
trying to retrieve an item.
pub fn invalidate_entries_if<F>(
&self,
predicate: F
) -> Result<PredicateId, PredicateError> where
F: Fn(&K, &V) -> bool + Send + Sync + 'static,
pub fn invalidate_entries_if<F>(
&self,
predicate: F
) -> Result<PredicateId, PredicateError> where
F: Fn(&K, &V) -> bool + Send + Sync + 'static,
Discards cached values that satisfy a predicate.
invalidate_entries_if
takes a closure that returns true
or false
. This
method returns immediately and a background thread will apply the closure to
each cached value inserted before the time when invalidate_entries_if
was
called. If the closure returns true
on a value, that value will be evicted
from the cache.
Also the get
method will apply the closure to a value to determine if it
should have been invalidated. Therefore, it is guaranteed that the get
method must not return invalidated values.
Note that you must call
CacheBuilder::support_invalidation_closures
at the cache creation time as the cache needs to maintain additional internal
data structures to support this method. Otherwise, calling this method will
fail with a
PredicateError::InvalidationClosuresDisabled
.
Like the invalidate
method, this method does not clear the historic
popularity estimator of keys so that it retains the client activities of
trying to retrieve an item.
Returns the max_capacity
of this cache.
Returns the time_to_live
of this cache.
Returns the time_to_idle
of this cache.
Returns the number of internal segments of this cache.
Cache
always returns 1
.
Trait Implementations
Auto Trait Implementations
impl<K, V, S = RandomState> !RefUnwindSafe for Cache<K, V, S>
impl<K, V, S = RandomState> !UnwindSafe for Cache<K, V, S>
Blanket Implementations
Mutably borrows from an owned value. Read more